The standard technology for making master discs for optical media uses a lithographic process based on a photoresist. For example, in a manufacturing method for a master disc, a photoresist layer on a substrate is exposed by a radiation beam: a photoresist mastering process is generally based on a photochemical process that takes place in the photoresist when it is illuminated with a laser beam. Every photon entering the photoresist has a certain chance of inducing a chemical change in the photoresist. This implies that every photon in the writing spot can result in a chemical change and that the region where chemical changes take place is theoretically infinite. However, chemical changes in the photoresist may cumulate when the photoresist receives multiple exposure doses. This means that earlier induced chemical changes may increase due to intersymbol interference during mastering.
Master discs manufactured on the basis of an optical process may also be used as a basis for stampers for the mass replication of read-only memory (ROM) and pre-grooved write-once (R) and rewritable (RE) discs.
During manufacturing, a thin photosensitive layer on a substrate can be illuminated with a laser beam to obtain exposed areas of the layer. Further, the exposed areas may be dissolved in a development process to form physical holes in the photoresist layer.
Recently, phase-transition mastering (PTM) has become a new method to make high-density ROM and RE/R stampers for mass-fabrication of optical information carriers. In particular, phase-transition materials can be transformed from their initial unwritten state to a different state via laser-induced heating. Heating can, for example, cause mixing, melting, amorphisation, phase-separation, decomposition, etc. in the material or layer. One of the two phases, the initial or the written state, dissolves faster in acids or alkaline developer liquids than the other phase does. Due to this characteristic behavior, a written data (information) pattern can be transformed to a kind of relief structure with protruding bumps or pits. The patterned carrier can then be used as stamper for mass-fabrication of high-density optical discs or as stamp for micro-contact printing.
One of the challenges encountered with PTM is to obtain a good pit shape. Since the whole method is based on heating, the shape might be determined by the temperature profile in the recording stack. The problem lies in the fact that most materials have either a rather high thermal conductivity (e.g. most metals) or a rather low thermal conductivity (e.g. most dielectrics). However, materials with a high thermal conductivity often have a bad optical absorption profile. While the heat is penetrating the stack of layers, the thermal conductivity gives a rapid cooling and thus a rapid decrease in the maximum temperatures that are reached locally. This makes it difficult to get the needed pit depth. Materials with a low thermal conductivity have a more localized heat built-up (which may be advantageous for a good pit shape), but getting the needed temperatures requires high writing powers when the optical absorbance is low.
It is therefore an object of the present invention to provide a master disc and a method for producing a master disc having a good pit shape.
The above objects are achieved by the features of the subject-matter of the claims.